2. Description of the Related Art
Location of features in semiconductor microcircuits for failure analysis (FA) work has often been a difficult task. With advancing technology the size of features of interest for FA has decreased. Traditional methods of making electrical contact to features of interest in FA, also called probing, involved using mechanical positioners with fine probing needles and an optical microscope. The positioners are precision, 3-axis stages that can be manual or motorized. Attached to the positioners are sharp, probing needles. Using a traditional optical microscope and the positioner, a user would probe the FA device of interest with the needle. The small size of current semiconductor technology has made location and probing of FA features difficult or even impossible because of the limits of optical microscopy.
Scanning probe microscopy (SPM) is one technique that can be used to locate these features. SPM can be used to create and image and locate features of interest that are much smaller than features that could be located using traditional optical microscopy. Since SPM can probe only one FA feature of interest per scanning probe microscope, multiple scanning probes are needed to contact multiple features. The field of using SPM, also called Atomic Force Microscopy (AFM), for the purpose of FA probing is called Atomic Force Probing (AFP). The acronym AFP is used to describe the field as well as instruments designed for use in the field, Atomic Force Probes.
The prior art contains many examples of using SPM to locate FA features including using a single SPM to locate FA features. However, only a limited number of FA experiments can be performed with a single probe as many devices of interest for FA require 2 probes, in the case of diodes, 3 probes, in the case of transistors, or even more probes.
The limited prior art relating to multiple scanning probes for FA shows the probes scanning one at a time in order to avoid collisions. This method is effective at avoiding collisions, at least until the probes move to their respective features of interest. However, this method takes longer to perform the scanning. This allows more time for drift, such as thermal drift, to occur. Also, the simple fact of longer measurement time is a serious weakness of the prior art.
One embodiment of the prior art for 2 probes is shown pictographically in FIGS. 1a-1d. A sample 110 contains features of interest 112. These features of interest 112 may be too small to be probed easily using traditional methods. FIG. 1a shows the scanning probe tips 114 grossly positioned on the sample 110 and near the features of interest 112. Each scanning probe tip 114 will scan and image a scan area 116. FIG. 1b shows the first scanning probe tip 114 starting at a start point 118 and scanning its area of interest 116. FIG. 1c shows the same process for the second scanning probe tip 114. At any given point during scanning the scan direction 120 for the different scanning probe tips 114 may or may not be the same direction. FIG. 1b and FIG. 1c show different scanning directions, as is common in the prior art. FIG. 1d shows the scanning probe tips 114 positioned on the features of interest 112 and ready for an FA experiment. This process took twice the amount of time as was required for one scan. Similarly, if more probes are needed in the experiment the time delay scales with the number of probes used.
This process also requires a difficult initial gross positioning setup. When the scanning probes tips are initially placed, they must be sufficiently far apart so that when one scanning probe tip is scanning, it does not collide with any other scanning probe tip that is not scanning. This requires initially placing the scanning probe tips sufficiently far apart to avoid the collision, and makes placing the scanning probe tips sufficiently close together to scan and image the same area difficult.
It is, therefore, desirable to perform SPM using multiple probes and provide scanning of multiple probes in the same amount of time as would be taken to perform a scan of a single probe. This provides the advantage of less time for SPM drift effects, as well as the simple advantage of less measurement time. This also provides the advantage of a more simple and efficient initial gross positioning setup, because collision with scanning probe tips that are not scanning does not need to be avoided.